Drill Wear: PCB Drilling and Hole Quality: What Controls the Result

The quality of a drilled hole determines the quality of everything that happens inside it, and the factors that control it are divided between the designer, who sets the geometry, and the shop, which sets the process. Understanding where the boundary lies avoids arguments about defects that neither party can fix alone.

Hole Size, Finished and Drilled

A plated hole is drilled oversize and then reduced by plating, so the drilled diameter is larger than the finished diameter by roughly twice the plating thickness. On a hole of half a millimetre that difference is a meaningful fraction of the size, which is why a drawing that quotes a finished size and a shop that works from a drilled size will produce a component that does not fit.

The relationship is straightforward but the consequences are not, because the plating thickness varies with the current distribution across the panel. A hole surrounded by dense copper plates differently from one in an isolated area, and the difference is one reason why the tolerance on a finished hole size is wider than the tolerance on the drill itself. Our plating thickness notes describe how the deposit is controlled.

Drill Wear and Its Symptoms

A drill wears as it cuts, and a worn drill produces a hole that is oversize at the entry surface and tapered along its length, because the cutting edges have lost their geometry. The wear is tracked by the number of hits rather than by time, and the limit depends on the material and on the hole size.

The symptom is a hole that passes the finished-size check at the top and fails at the bottom, which is why the measurement is normally made on a microsection rather than with a pin gauge. A pin gauge that enters from the entry side can report a hole as acceptable when the barrel is undersized.

Burrs, Nails and Entry Materials

Drilling copper produces a burr at the exit surface if the material is not supported, and a burr that is not removed will interfere with the imaging step and can detach later, leaving a particle inside the equipment. The remedy is an entry material on top and an exit material below, both chosen so that they do not themselves contaminate the hole.

An exit burr that remains attached is often called a nail, and it is a particular problem on thick copper because the burr is larger. Where a design uses heavy copper, the drilling parameters and the entry material are adjusted together rather than independently.

Hole Wall Quality and Desmear

After drilling, the resin in the hole wall is smeared by the cutting action, and that smear has to be removed before the plating can adhere. The removal is done chemically or by a plasma process, and the amount of material removed is controlled so that the glass fibres within the laminate are exposed without being damaged.

Where the smear is not removed completely, the plating adheres to the smear rather than to the laminate, and the connection between the copper and the hole wall fails during thermal cycling. The resulting defect is an intermittent open that appears after the product has been in service. Our hole copper notes describe how the wall is inspected.

<img src="https://www.gopcba.com/wp-content/uploads/2026/05/智能货架2.jpg" alt="Microsection of plated holes showing annular ring” />

Aspect Ratio and Drilling Depth

Aspect ratio is the board thickness divided by the hole diameter, and it describes how difficult the hole is to drill and plate. A high aspect ratio means a long, narrow hole from which the debris is hard to clear and into which the chemistry penetrates slowly.

The practical limits are set by the shop rather than by the material. A ratio of ten to one is routine, twenty to one is achievable with care, and beyond that the process becomes specialised and the yield falls.

Backdrilling and Stub Control

Where a via carries a signal at high frequency, the unused portion of the barrel acts as a stub that reflects energy at a frequency determined by its length. Backdrilling removes that stub by re-drilling the hole from the far side to a controlled depth.

The depth control determines whether the stub is removed without cutting into the layers that carry the signal, and it depends on the thickness tolerance of the board and on the drilling machine’s accuracy. Our aspect ratio notes describe the related geometry.

Debris and Hole Cleanliness

Resin debris left in a hole reduces the plating adhesion and can produce a void in the barrel. The debris is removed by the desmear process, but the process has to reach the whole depth of the hole, which is another reason why a high aspect ratio is difficult.

Cleanliness is verified by sectioning a sample and examining the barrel, and the acceptance criterion is usually expressed as a maximum void percentage in the plated copper rather than as a visual description.

What the Designer Controls

The designer sets the hole size, the aspect ratio, the annular ring and the distance from a hole to the nearest plane or conductor. A hole that is too close to a plane will be affected by the copper distribution during plating, and a hole with a thin annular ring leaves no margin for the registration error that the drilling and lamination processes produce.

The simplest rule is to give the hole more annular ring than the minimum requires, because the cost of the extra ring is a little board area while the cost of the defect is a scrapped panel. Our hole types notes describe the standard relationships.

Process Control and Verification

Reviewing the design before the data is released is far cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

FAQ

Why do holes come out smaller than the drawing? Because the drawing normally states a finished size and the shop drills to a larger diameter that is reduced by plating. Where a hole comes out undersized, the cause is usually an insufficient drilled diameter or an excess of plating, and the two are distinguished by measuring the plating thickness on the accompanying coupon.

How is hole wall quality checked? By sectioning a sample hole and examining the plated copper along its length, looking for voids, cracks and separation between the copper and the laminate. The result is recorded against a defined acceptance criterion rather than judged by appearance.

What does gopcb specify for a hole table? We ask for the finished size, the tolerance, the plating condition and the quantity for every tool, and we state our drilled diameters in the fabrication review. That exchange removes the ambiguity that causes most hole-related queries.